It is very difficult to copolymerize vinyl acetate with several commercially important monomers, especially in the presence of ethylene. While methyl methacrylate (MMA) can be polymerized with vinyl acetate, the level of MMA with which this occurs is insufficient to provide the copolymer with the properties associated with MMA, such as block resistance and solvent tensiles in a nonwoven copolymer binder. Usually a level of 10% MMA stops the vinyl acetate conversion and lower levels of MMA are usually insufficient to afford low blocking.
Copolymerizing vinyl acetate with styrene is very difficult, if at all possible. Even the preparation of a stable blend of polyvinyl acetate and polystyrene emulsions is difficult to obtain.
U.S. Pat. No. 4,616,057 discloses the preparation of an aqueous polymer emulsion containing an interpenetrating polymer network. This reference teaches a method of polymerizing MMA and styrene in a VAE copolymer emulsion. A multi-functional active crosslinker is required in the first stage polymerization and the second stage monomers must be equilibrated with the first stage emulsion before they are polymerized. The examples show that additional surfactants are added with the second stage monomers for this emulsification step.
U.S. Pat. No. 4,683,165 discloses aqueous interpenetrating polymer network emulsions containing a first vinyl acetate-ethylene copolymer network which is intertwined on a molecular scale with a second polystyrene or polymethyl methacrylate network. This patent is a continuation-in-part of the '057 patent.